4.8is fuel system blues

[SocalD]

Hey all! I have been chasing a fuel related issue for several weeks now and it is driving me nuts! It all started when my 2005 X5 4.8is died on the freeway and would not restart. Prior to this happening I had some strange things happening. For one I would hear this strange "whiney" sound coming from the driver side rear passenger seat which was likely the fuel transfer jet pump making the noise. The noise would get higher in pitch and start to fade away after 10sec and then disappear. Occasionally the engine would "stumble" at idle and almost die but would regain itself after a second. And when driving the sometime the acceleration would be poor and even slightly hesitate in first gear but the after a while would magically go away and have great acceleration. So I suspected the fuel pump was bad and sure enough it was; put 12V directly to the fuel pump and motor did not run. After a replacing the fuel pump, it pretty much started right up. After driving a couple of times the whiney sound returned although not as often and also the "stumbles". And I thought, hmm must be the fuel filter and pressure regulator since I did not replace. So then I replaced the fuel filter. Ran the car again and did not change much, still have occasional whiney sound, randomly poor acceleration, and although the car does not stumble, the tone of the idle deepens as if having fuel flow problems but then improves after a while. One thing to note, the car loses fuel pressure so when I go to restart the engine, it does not start right away. And I took off the fuel hose to see if any backpressure after running, and barely any fuel trickled out; I noticed a ton of gas came out for other people on YouTube who replaced the fuel pump and disconnected the fuel line. So now I am at a loss! It almost seemed like it is the fuel pressure regulator but I just changed the fuel filter which has the built in pressure regulator. Could it be a leaking injector? And this is a stretch but could the MAF be defective and sending the wrong fuel demand? Scratching my head...